package com.zhang.study.chapter10;

import java.util.Stack;

/**
 * 判断是否是回文链表
 */
public class Code02_IsPalindromeList {

    public static class Node {
        public int value;
        public Node next;

        public Node(int data) {
            this.value = data;
        }
    }

    /**
     * 判断是否是回文链表核心方法
     *
     * @param head 头结点
     * @return 是否是回文链表
     */
    public static boolean isPalindrome1(Node head) {
        if (head == null) {
            return true;
        }
        Stack<Node> stack = new Stack<>();
        Node cur = head;
        while (cur != null) {
            stack.push(cur);
            cur = cur.next;
        }
        cur = head;
        while (cur != null) {
            if (cur.value != stack.pop().value) {
                return false;
            }
            cur = cur.next;
        }
        return true;
    }

    public static boolean isPalindrome2(Node head) {
        if (head == null) {
            return true;
        }
        Node n1 = head;
        Node n2 = head;
        while (n2.next != null && n2.next.next != null) {
            n1 = n1.next;
            n2 = n2.next.next;
        }
        // n1 是中点位置
        Node n3 = null;
        n2 = n1.next;
        n1.next = null;
        while (n2 != null) {
            n3 = n2.next;
            n2.next = n1;
            n1 = n2;
            n2 = n3;
        }
        // n1 是最后一个节点
        n3 = n1;
        n2 = head;
        boolean res = true;
        while (n1 != null && n2 != null) {
            if (n1.value != n2.value) {
                res = false;
                break;
            }
            n1 = n1.next;
            n2 = n2.next;
        }
        n1 = n3.next;
        n3.next = null;
        while (n1 != null) {
            n2 = n1.next;
            n1.next = n3;
            n3 = n1;
            n1 = n2;
        }
        return res;
    }

    public static void main(String[] args) {
        Node head = null;
        head = new Node(1);
        head.next = new Node(2);
        head.next.next = new Node(3);
        head.next.next.next = new Node(2);
        head.next.next.next.next = new Node(1);
        System.out.println(isPalindrome1(head));
        System.out.println(isPalindrome2(head));
        head = new Node(1);
        head.next = new Node(1);
        head.next.next = new Node(3);
        head.next.next.next = new Node(2);
        head.next.next.next.next = new Node(1);
        System.out.println(isPalindrome1(head));
        System.out.println(isPalindrome2(head));
    }
}
